Long Term Evolution (LTE) and High-Speed Packet Access (HSPA) cellular radio communication systems are sometimes called “third generation” (3G) systems and are currently being standardized by the Third Generation Partnership Project (3GPP). The LTE specifications can be seen as an evolution of the current wideband code division multiple access (WCDMA) specifications. An IMT advanced communication system (i.e., a “fourth generation” (4G) system) uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, HSPA, or other communication system for IMS multimedia telephony (IMT). The 3GPP promulgates the LTE, HSPA, WCDMA, and IMT specifications, and specifications that standardize other kinds of cellular wireless communication systems.
FIG. 1 depicts a typical cellular radio communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate base station(s) (BSs), which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or eNodeB in LTE vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the eNodeBs 22, 24, 26, and other functionalities can be moved to other nodes in the network. It will also be understood that a base station can use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas can send respective, different pilot signals.
An LTE system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs). An LTE system has channel bandwidths ranging from about 1 MHz to 20 MHz, and supports data rates up to 100 megabits per second (Mb/s) on the largest-bandwidth channels. One type of physical channel defined for the LTE downlink is the physical downlink shared channel (PDSCH), which conveys information from higher layers in the LTE protocol stack and is mapped to one or more specific transport channels. The LTE physical layer, including the PDSCH and other LTE channels, is described in 3GPP Technical Specification (TS) 36.211 V8.7.0, Physical Channels and Modulation (Release 8) (June 2009), among other specifications.
In an OFDMA communication system like LTE, the data stream to be transmitted is portioned among a number of narrowband subcarriers that are transmitted in parallel. In general, a resource block devoted to a particular UE is a particular number of particular subcarriers used for a particular period of time. A resource block is made up of resource elements (REs), each of which is a particular subcarrier used for a smaller period of time. Different groups of subcarriers can be used at different times for different users. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a UE to demodulate each subcarrier. Like many modern communication systems, DL transmissions in an LTE system are organized into frames of 10 milliseconds (ms) duration, and each frame typically includes twenty successive time slots. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
For cell measurements, channel estimation, and other purposes, reference symbols or signals (RS), which may be called pilots, are transmitted from each eNodeB at known frequencies and time instants. RS are described for example in Sections 6.10 and 6.11 of 3GPP TS 36.211, and are transmitted from each of possibly 1, 2, or 4 transmit antennas of an eNodeB on particular REs that can be conveniently represented on a frequency-vs.-time plane as depicted in FIG. 2. It will be understood that the arrangement of FIG. 2 is just an example and that other arrangements can be used.
FIG. 2 shows an arrangement of subcarriers in resource blocks in two successive time slots, which can be called a sub-frame, in an LTE system. The frequency range depicted in FIG. 2 includes twenty-seven subcarriers, only nine of which are explicitly indicated. In FIG. 2, the resource blocks, which are indicated by dashed lines, each include twelve subcarriers spaced apart by fifteen kilohertz (kHz), which together occupy 180 kHz in frequency and 0.5 ms in time, or one time slot. FIG. 2 shows each time slot including seven OFDM symbols, or REs, each of which has a short (normal) cyclic prefix, although six OFDM symbols having long (extended) cyclic prefixes can be used instead in a time slot. It will be understood that resource blocks can include various numbers of subcarriers for various periods of time.
RS transmitted by a first transmit (TX) antenna of an eNodeB are denoted R and by a possible second TX antenna in the node are denoted by S. In FIG. 2, RS are depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 4 (because the symbols have short cyclic prefixes) in every slot. Also in FIG. 2, the RSs in symbols 4 are offset by three subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
Besides reference signals, predetermined synchronization signals are provided for a cell search procedure that is a UE carries out in order to access the system, or network. The cell search procedure includes synchronizing the UE's receiver with the frequency, symbol timing, and frame timing of a cell's transmitted signal, and determining the cell's physical layer cell ID. The cell search procedure for an LTE system is specified in, for example, Section 4.1 of 3GPP TS 36.213 V8.6.0, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Layer Procedures (Release 8), June 2009.
LTE uses a hierarchical cell search scheme similar to WCDMA, in which eNodeB-UE synchronization and a cell group identity (ID) are obtained from different synchronization channel (SCH) signals. A primary synchronization channel (P-SCH) signal and a secondary synchronization channel (S-SCH) signal are defined with a pre-defined structure in Section 6.11 of 3GPP TS 36.211. For example, P-SCH and S-SCH signals can be transmitted on particular subcarriers in particular time slots. In an LTE system, the eNodeBs transmit two different synchronization signals: a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) that are transmitted on a 5-ms basis in each cell.
FIG. 2 shows the SSS and PSS as OFDM symbols 5, 6 (assuming operation with the short cyclic prefix and frequency-division duplex (FDD). Current LTE systems have the PSS and SSS symbols transmitted in the middle six resource blocks in sub-frames 0 and 5. Comparable reference and synchronization channels are often provided in other digital communication systems, although they may be given different names.
The PSS exists in three versions, one for each of three cell-within-group IDs, and is based on Zadoff-Chu (ZC) sequences that are mapped onto the central 62 REs. ZC sequences are a special class of generalized chirp-like (GCL) sequences. A ZC sequence having a length N, where N is odd, and a sequence index u is defined by the following expression:Zu(k)=exp(−j·π/N·u·k·(k+1)),k=0,1, . . . ,N−1.The three different PSS signals in LTE are ZC sequences of the same length N with different sequence indices u. The PSS and SSS and aspects of ZC sequences and synchronization are described in U.S. Patent Application Publication No. US 2008/0267303 A1 by R. Baldemair et al.
There are in total 168 cell groups, and the SSS carries information, which is based on m-sequences, on which cell group a cell belongs to. The SSS also carries information on whether it is transmitted in subframe 0 or subframe 5, which is used for acquiring frame timing. For a particular cell, the SSS is scrambled with the cell's cell-within-group ID, and so in total there are 2×504 versions of the SSS, two for each of the 504 physical layer cell IDs. Like the PSS, the SSS is mapped onto the central 62 REs.
Before the UE has found its first cell, the UE acquires frequency synchronization by not only tuning its receiver to the frequency of the carrier signal transmitted by an eNodeB, but also finding any undesirable offset between the cell's carrier frequency and the frequency of the oscillator or signal generator used by the UE for demodulating its received signal. Methods and apparatus for determining and using frequency offsets are described in, for example, U.S. Patent Application Publication No. US 2008/0013650 by K. Engdahl and U.S. Pat. No. 7,443,826 to R. Atarius et al.
After frequency synchronization, cell search typically involves the UE's correlating its received signal with its local replicas of the three versions of the PSS, e.g., using a matched filter. The period of the correlation usually includes symbols received during at least 5 ms. Correlation signal peaks in the matched-filter output are used to acquire symbol synchronization, and can reveal synchronization signals from one or more cells.
After frequency and symbol synchronization using the PSS, the UE knows the position of the SSS and proceeds to decode the SSS to acquire frame timing and determine the cell's group ID. The information about which of the three PSS versions was received and the cell's group ID establishes the physical layer cell ID of the cell. The UE then has all the information it needs to read broadcast system information and establish communication with the cell. Moreover, cyclic prefix configuration and potentially even duplex mode is determined. SSS position determination and decoding is sometimes called SSS Detection in this application.
A frequency offset between the cell and the UE can arise when the UE's oscillator in its demodulator is ill-tuned. In general, the less expensive the UE's oscillator is, the wider its tuning tolerance is, and thus the larger the potential offset is when the UE is powered on. The PSS is robust against offset between a cell's carrier frequency and the UE's demodulation frequency. Under favorable radio conditions, it is possible to detect the PSS and its timing even if the frequency offset is as large as ±7.5 kHz, which is half the subcarrier spacing in an LTE system. As the SSS carries more information than the PSS, the SSS is more sensitive than the PSS to frequency offset. Hence, before detecting the SSS, the frequency offset has to be estimated/detected and mitigated.
It is known in the art that the frequency offset can be estimated by correlating the received PSS with one or more local copies of the PSS sequence over a grid of frequency offset hypotheses. That estimation can be implemented either by re-tuning the UE's oscillator to each frequency in the grid or by digitally shifting the correlators, or matched filters. The matched filter yielding the largest correlation metric (e.g., peak magnitude) is then considered to indicate both the cell ID (from the PSS version used) and the frequency offset (from the frequency shift of the filter or the oscillator). Such a frequency offset estimate can be refined in a number of ways, e.g., by using a denser grid of offset hypotheses, or by first using a coarse grid of hypotheses and then using a dense grid at timings detected with the coarse grid, among other ways. The invention described in this application is independent of the strategy chosen for frequency offset refinement. Correlating a received signal against local PSS replicas with or without frequency offset hypotheses is sometimes called PSS Detection in this application.
The time-domain correlation properties of the ZC sequences upon which the PSS is based are robust against small frequency errors as noted above, and so even if there is a small frequency offset, a peak in the PSS matched filtering still accurately indicates the position of the received PSS. Nevertheless, if the frequency offset is larger than about ±22.5 kHz, spurious correlation peaks of substantial magnitude can arise at positions other than the start of the received PSS symbol. In the presence of noise, those spurious peaks can be as large as or even larger than the correlation peak at the correct timing and frequency offset. As a result, when testing frequency offset hypotheses, the UE can erroneously deduce that it has tuned in to the correct carrier frequency (within ±7.5 kHz), by which the UE will assume an incorrect symbol timing, and ultimately will find an incorrect physical layer cell ID after decoding the SSS. Depending on how a UE is implemented, if those errors go undetected, they can result in wasted radio resources and a longer initial cell search and even a failed cell search in unfavorable radio conditions.
Many previous approaches to mitigating the problems arising from large frequency offset errors involve the use of a high-quality oscillator with low tolerances such that even before tuning, the frequency offset never exceeds about ±20 kHz. Those approaches are undesirable for several reasons, e.g., because high-quality oscillators are expensive and can increase the complexity and energy consumption of the UE.